NOx formation in two-stage methane-air flames

To help understand how staged combustion aids in reducing emissions of oxides of nitrogen from gas turbines, measurements and computations are made of structures of two-stage counterflow methane-air flames at normal atmospheric pressure and a feed-stream temperature of about 300 K. The fuel stream is partially premixed, with equivalence ratios from 1.5 to 3.0. To the air stream is added up to 10% by mass of water spray, carbon dioxide, or nitrogen. Flame structures, including formation of species containing two carbon atoms, are measured by gas chromatography of samples withdrawn by fine quartz probes and are calculated by numerical integrations of the conservation equations employing an updated elementary chemical-kinetic data base. The same sampling system is employed with a low-flow-rate NOx analyzer to obtain profiles of nitric oxide and nitrogen dioxide, which are also calculated in the numerical integrations. The two-stage flame exhibits a green fuel-rich premixed flame and a blue diffusion flame with the maximum NOx concentrations found near the blue flame. At an air-side strain rate of 50 s(-1), for fuel-side equivalence ratios of 1.5, 2.0, 2.5 and 3.0, respectively, measured peak NOx concentrations were about 70, 90, 100, and 90 ppm, reduced to 60, 70, 50, and 40 ppm, respectively, when 5% water by mass was added to the air stream. Results of the numerical integrations were in improved agreement with these experimental results when suitable selections were made of certain critical elementary reaction-rate constants. These new NOx measurements and computations help to increase understanding of influences of staging and diluent addition, identify important reactions for pollutant formation and suggest means to reduce emissions. (C) 1999 by The Combustion Institute.